The present invention relates to adsorbents having the capability of decomposing organic halogen compounds and being adapted to be used mainly for treating waste gases from incinerators and also to a process for producing the same.
As measures for reducing organic halogen compounds generated from incinerators for municipal waste and industrial waste, improvements in combustion conditions and advances in waste gas treatment have been attempted. In general, the temperature must be maintained at as high as 1000xc2x0 C. or more for combustion processes to completely decompose chlorinated organic materials like organic halogen compounds through oxidation. In order to prevent damage of furnace walls, the combustion temperature is commonly set at 800 to 900xc2x0 C., and therefore it is not easy to completely decompose the organic halogen compounds through oxidation by improving the combustion conditions.
In stoker furnaces, in which locally low-temperature portions or locally incomplete combustion portions are likely to be formed, a re-combustion portion needs to be provided to completely decompose the organic halogen compound through oxidation. However, there are still problems of an increase in the total waste gas amount and an increase in the running cost due to a re-combustion fuel.
Further, it is known that among the organic halogen compounds, dioxins and brominated dioxins are not only produced during a combustion process, but they are also synthesized again from remaining unburnt materials that are not completely burnt and precursor materials having benzene rings in a wide temperature ranges of about 300 to 500xc2x0 C. Thus, even if the organic halogen compounds such as dioxins and brominated dioxins are intended to be reduced in the combustion process, the organic halogen compounds such as dioxins and brominated dioxins may be synthesized again during the succeeding stage, so that the organic halogen compounds including dioxins and brominated dioxins may not be reduced in the waste gases.
For the above reasons, the organic halogen compounds including dioxins and brominated dioxins are required to be reduced by treating the waste gases so that the organic halogen compounds may be stably maintained at a low concentration in the waste gases.
Among methods for removing the organic halogen compounds including dioxins and brominated dioxins by treating the exhaust gases, an adsorbing method by which such organic halogen compounds are adsorbed with activated carbon is mainly employed. However, the activated carbon having adsorbed the organic halogen compounds becomes a secondary waste product, which requires post treatment thereof.
In order to solve this problem, the present inventors had been investigating prolonging the span life span of activated carbon, particularly when used in combination with a V2O5xe2x80x94TiO2 based catalyst or a V2O5xe2x80x94WO3xe2x80x94TiO2 based catalyst known as a catalyst for decomposing organic halogen compounds including dioxins and brominated dioxins. However, it was confirmed that if the concentration of dioxins and brominated dioxins (as kinds of organic halogen compounds) is low at a catalyst inlet in the case of the V2O5xe2x80x94TiO2 based catalyst or the V2O5xe2x80x94WO3xe2x80x94TiO2 based catalyst, the dioxins and the brominated dioxins are inversely produced. Recently, considering in many areas that the concentration of dioxins in the waste gas is suppressed to not more than {fraction (1/10)} of the governmental standard value by residential agreements, it was clarified that use of such conventional organic halogen compound-decomposing catalysts might afford adverse effects on the above considerations.
Moreover, since conventional organic halogen compound-decomposing catalysts need to be used at high temperatures of 200 to 500xc2x0 C., the waste gases must be heated again. Further, it was clarified that because the activated carbon cannot withstand such high temperatures, the catalyst and activated carbon cannot be used in the same position, and it is not easy to use the catalyst and the activated carbon in combination.
The present invention has been accomplished to solve the conventional problems as mentioned above, and to provide an adsorbent having the capability of decomposing organic halogen compounds. The adsorbent not only adsorbs organic halogen compounds contained in waste gases, but also effectively decomposes them to prolong the life span of the adsorbent and to suppress the amount of a secondary waste produced, while having no ability to re-synthesize the organic halogen compounds (including dioxins and brominated dioxins) at that time. The invention also provides a process for producing the same.
The adsorbent having the organic halogen compound-decomposing function according to the present invention, which has been made to solve the above problems, comprises an activated carbon and an organic halogen compound-decomposing catalyst carried on the activated carbon. The organic halogen compound-decomposing catalyst has fine particles of Pt and/or Pd dispersed in TiO2 by mixing a dispersion liquid of an organic metal precursor containing Pt and/or Pd and Ti in molecules thereof with a sol of TiO2 and producing the catalyst under heating.
A process for producing a catalyst having a capability of decomposing an organic halogen compound according to the present invention comprises the steps of reacting a salt of Pt and/or Pd, an organic titanium compound and an organic binder in a solvent, to thereby synthesize a dispersion of an organic metal compound precursor containing Pt and/or Pd and Ti in molecules thereof; mixing a sol of TiO2 with the dispersion to prepare a catalyst-coating solution; impregnating an activated carbon with the catalyst-coating solution; and heating the resulting mixture, thereby allowing the activated carbon to carry thereon an organic halogen compound-decomposing catalyst comprising TiO2 and, dispersed therein, fine particles of Pt and/or Pd. As the organic binder, an amino acid such as L-lysine or L-proline is preferably used. As the organic titanium compound, an alkoxide derivative of Ti is preferably used.
The adsorbent having the capability. of decomposing the organic halogen compound according to the present invention is produced through mixing the sol of TiO2 with the dispersion of the organic metal compound precursor containing Pt and/or Pd and Ti in molecules thereof, followed by heating, thereby allowing the activated carbon to carry thereon the organic halogen compound-decomposing catalyst comprising TiO2 having fine particles of Pt and/or Pd dispersed therein. Since the number of activated points is extremely large, that the catalyst can be highly activated. Therefore, while the amount of expensive Pt and Pd is reduced, the organic halogen compound can be decomposed. The mixture of the organic metal compound precursor dispersion and the TiO2 sol is heated at a temperature of approximately 450xc2x0 C.-500xc2x0 C. in an air atmosphere under ordinary atmospheric pressure. The heating time is set at such a time in connection with the heating temperature, that the organic binder may be completely burnt out and that Pt and/or Pt may be uniformly dispersed in TiO2.
Further, the organic halogen compound is not synthesized again, unlike the conventional V2O5xe2x80x94TiO2 based catalyst or the V2O5xe2x80x94WO3xe2x80x94TiO2 based catalyst. In addition, the adsorbent exhibits an excellent function at the use temperature of the activated carbon of approximately 130-150xc2x0 C., so that the useful life of the activated carbon can be prolonged to a few times longer than that of the conventional adsorbent, while the waste gases need not be re-heated.